On-vehicle radars have been developed, which utilize radio waves of a millimeter waveband, for example, for measuring the distance, speed and azimuth, for example, associated with a forward target, for application to inter-vehicle control or mitigation of collision between vehicles. Recently, radars are required, for example, to track an obliquely forward vehicle as viewed from an own vehicle when passing through a curve at low speed, or detect obstacles near the own vehicle. Under such circumstances, there is a need for radars having an azimuth detection performance which enables monitoring over a wide horizontal range (wide angle). In response to such a demand, electronically scanned radar systems have gradually been adopted, which systems utilize array antennas structured by a plurality of antenna elements.
What is important for monitoring a distanced region is azimuth resolution of a radar. For example, azimuth resolution of several degrees is required in order to dissociate and detect two vehicles traveling alongside some 100 m forward along two respective lanes. Generally, enhancement of the azimuth resolution can be achieved by broadening the aperture of the array antenna. However, broadening the aperture of an array antenna yields a problem of increasing the size of the radar.
Also, in the case where azimuth detection is carried out by digital beam forming (DBF) using an array antenna having uniformly spaced elements, for example, grating lobe may be caused depending on the size of the space between the array antennas used for reception. The grating lobe occurs when the phase of a reception signal is turned by 2nπ. Therefore, an angle avoiding an azimuth that may cause the grating lobe is established as an azimuth that enables detection.
For example, as shown in FIG. 1, there may be a case where azimuth detection is performed for a preceding object, or a vehicle 61, by conducting digital beam forming (DBF) with the aid of an array antenna with uniformly spaced elements, which is loaded on a vehicle 60. In this case, as shown in FIG. 6, some peaks are caused by objects that are present on both sides of the road. In particular, a peak PK1 may be caused, which corresponds to a reflected wave S1 that is a reflection from a roadside object 62, a peak PK3 may be caused, which corresponds to a reflected wave S3 that is a reflection from a roadside object 63, and a peak PK2 may be caused, which corresponds to a reflected wave S2 that is a reflection from the vehicle 61. Besides these peaks, grating lobes PKN1, PKN2 and PKN3 may be caused.
The azimuth of causing these grating lobes PKN1, PKN2 and PKN3 is substantially in proportion to the inverse of the antenna space. Accordingly, when DBF is conducted using a uniform array antenna, the grating lobes PKN1, PKN2 and PKN3 can be removed out of the monitoring range (scanning range of the radar) by reducing (narrowing) the antenna space, to thereby broaden the angle of detection.
Being based on this idea, a uniform array antenna having a small space between the antenna elements and a wide antenna aperture may be used to realize an electronically scanned radar of high resolution, which is able to cover a wide horizontal range. However, such an array antenna is not practical because it involves increase in the cost due to the increase of the number of elements, or depression in the gain per antenna element.
Under the circumstances, a suggestion has been made to provide non-uniform spaces between the elements of an array antenna without enlarging the area of the antenna aperture, so that the occurrences of grating lobes can be suppressed and increase in the number of the elements or the size of the radar can be prevented as much as possible.
A technique concerning a method for structuring such a non-uniform array antenna is described, for example, in (Non-patent Document 1). This document suggests a method for providing a configuration in which the spaces between elements are rendered to have a prime number ratio so that the phases of the reception signals are differentiated, while digital beam forming (DBF) is used in performing azimuth detection.
Meanwhile, a high-resolution scheme, such as MUSIC, is known as a subspace method, replacing DBF, for enhancing the azimuth resolution. When a non-uniform array is used, however, in combination with the high-resolution scheme called subspace method, such as MUSIC, it is known that undesired peaks are also produced at azimuths that depend on the spaces between the array antenna elements and the incoming directions of the reception signals, in addition to the azimuths in which grating lobes of DBF occur.
[Non-Patent Document 1]
Paper titled “A Study on an Arrangement Spaces for a Non-uniform Array in Suppressing Gratinglobe”, by Osamu MIZOKAMI et al., Institute of Electronics, Information and Communication Engineers, January 2000, Vol. J83-B, No. 1, pp., 141-143